if you are using the mindcrack pack, you need to check the config files for thermalexpasion, that mods is controlling the ore generation. Im playing the same modpack and i have found plenty of tin, remember that you cant just go down to bedrock and mine everything, the ores are layered now.
MindCrack\minecraft\config\cofh\thermalexpansion.txt, go to the end of the txt and there you should see the ore generation.
/manu
